The present invention relates to a controller for use with an automatic transmission used in a power transmission mechanism in which driving force of a vehicle engine is converted for transmission to wheel shafts by means of the automatic transmission and more particularly, to an automatic transmission controller suitable for use with an automatic transmission provided with a direct coupling mechanism which mechanically transmits driving force of the engine to a transmission.
Some types of automatic transmission for vehicles using a torque converter adapted to transmit or couple driving force through the medium of fluid are provided with a lockup mechanism which transmits the driving force by mechanically coupling the engine output to the transmission without resort to intervention of fluid in order to improve fuel economy. In the lockup mechanism, the engine is directly coupled to the transmission by press force of clutch controlled by a working pressure. A halfway clutching state can be allowed by adjusting the working pressure on the clutch. Typically, with the aim of preventing a large shock from being generated owing to abrupt engagement of the clutch during a shift from fluid coupling state to direct coupling state, the working pressure is gradually increased to suppress the shock. Similarly, during a shift from direct coupling state to fluid coupling state, the working pressure is gradually decreased to relieve a shock.
In a conventional automatic transmission provided with the lockup mechanism, the initial value of working pressure during the shift from fluid coupling state to direct coupling state or vice versa is always kept to be constant regardless of various running conditions. Further, in the course of the shift between the direct coupling state and the fluid coupling state, the increasing and decreasing rates of the working pressure are constant.
JP-A-11-37279 discloses control of an automatic transmission having a continuously variable transmission and a lockup mechanism in combination. In the control described in this literature, the lockup state is released forcibly when the load decreases abruptly during the lockup. JP-A-4-175571 discloses that coupling force of a lockup clutch is so controlled as to be increased gradually. JP-A-3-260466 and JP-A-7-198034 disclose that coupling force is feedback-controlled such that a lockup clutch is brought into a slip state.
According to studies of the inventors of the present invention, it has been found that a working pressure necessary for engagement of a lockup clutch is liable to decrease when input torque (driving force) from the engine to the torque converter is small and conversely, a working pressure of the clutch necessary for lockup liable to increase as the input torque increases. Similarly, it has been found that during relief of the lockup clutch, a working pressure for relieving the lockup clutch is liable to decrease when the input torque is small and conversely, a working pressure necessary for relieving the lockup clutch is liable to increase as the input torque increases.
Accordingly, with the initial value of working pressure rendered to be constant and the increasing rate also rendered to be constant during the shift from the fluid coupling state to the direct coupling state as in the case of the conventional automatic transmission controller, the time ranging over the start of state shift and the completion of engagement is shortened when the input torque is small but is prolonged when the input torque is large. A similar phenomenon takes place during the shift from the direct coupling state to the fluid coupling state.
When the lockup time changes with the magnitude of the input torque, a problem is found to arise in which the driver has a large disordered feeling of immaturity of lockup or abnormally rapid maturity of lockup in contrast to natural maturity of the lockup.
Further, for example, under the condition that the input torque is small and the lockup is set up or established, much time is required for relieving the lockup clutch and therefore, an engine stall will take place when the driver applies abrupt braking or quick service.
Conceivably, measures may be taken to cope with the problems as above by increasing the increasing and decreasing rates of the working pressure during the state transition period. With the above countermeasures, however, the time for lockup engagement or relief is shortened when the input torque is large and a shock due to abrupt lockup engagement or relief increases, thus giving an uncomfortable feeling to the driver.
Moreover, in case the time for engagement of the lockup clutch is prolonged unnecessarily, the fuel economy is degraded.
According to the invention, in a control for use with an automatic transmission which is provided with a torque converter having a fluid coupling unit for coupling driving force to a transmission through fluid coupling and a direct coupling unit for coupling the driving force to the speed change gear through mechanical coupling and being operative to take a fluid coupling state, a direct coupling state or an intermediate state between the fluid and direct coupling states in accordance with a working pressure applied to the direct coupling unit, the controller comprises a working pressure controller for generating a control signal which changes the initial value of the working pressure in accordance with the driving force during the period of transition between the fluid coupling state and the direct coupling state.
According to the automatic transmission controller of the present invention, the period of transition between the fluid coupling state set up by the fluid coupling unit and the lockup state set up by the direct coupling unit, such as a lockup clutch, in the torque converter can extend for a proper time regardless of input torque and a shock during lockup clutch engagement or relief can be reduced to thereby improve the drivability.
Further, according to the invention, in a memory medium readable by a computer and storing a program for execution of a method of controlling an automatic transmission which is provided with a torque converter having a fluid coupling unit for coupling driving force to a transmission through fluid coupling and a direct coupling unit for coupling the driving force to the speed change gear through mechanical coupling and being operative to take a fluid coupling state, a direct coupling state or an intermediate state between the fluid and direct coupling states in accordance with a working pressure applied to the direct coupling unit, the program has a step of generating a control signal which changes the initial value of the working pressure in accordance with the driving force during the period of transition between the fluid coupling state and the direct coupling state.